1. Field of the Invention
This invention relates to a method of thin film forming on the semiconductor substrate, and more particularly to a method wherein ultra fine powder of metal dispersed in organic solvent is applied to the semiconductor substrate.
2. Description of the Prior Art
In order to get high density of packaging and high degree of integration, a multilayer wiring technique is widely used for semiconductor devices. FIG. 1 is a cross-sectional view of one typical example of multilayer wiring techniques. In this example, a silicon substrate 1 has a first layer 2 of silicon dioxide (SiO.sub.2) on one surface. The layer of SiO.sub.2 as passivation or insulating material is formed by the Chemical Vapor Deposition (CVD) technique. After the contact hole C which reaches a base wafer B, is bored through the layer 2 by the lithography technique, a first thin film of aluminum (Al) is formed by a sputtering technique, and then a patterned Al wiring 3 is formed by the lithography technique. Further, a second layer 4 of SiO.sub.2 is formed by the CVD technique. After the contact hole V which connects one wiring layer with another wiring layer, is bored through the layer 4 by the lithography technique, a second thin film 5 of Al is formed by the sputtering technique.
But, drawbacks are found in the sputtering process of Al to the SiO.sub.2 layer which has a contact hole or a via hole FIG. 2 is schematic drawings which give general idea of a common sputtering technique.
FIG. 2A shows flight directions f of Al particles in a sputtering chamber, and FIG. 2B shows thickness distribution of an Al film 14 formed at the contact hole C (similar to the via hole V) in the semiconductor substrate 12. As FIG. 2A shows, a substrate 12 is set apart from an Al target 11 at a distance in the range of 30 mm to 60 mm. A pressure in the chamber is kept in the range of 0.3 Pa to 0.5 Pa, and Al particles sputtered from the target 11 take flight in all directions as indicated by arrows f, They collide with each other repeatedly, and reach the substrate 12. Accordingly, as FIG. 2B shows, Al particles accumulate or deposit on the contact hole C and the SiO.sub.2 film. However, they often block entrance of the contact hole C in the substrate 12, resulting in a cavity Q in the contact hole C in spite of flat and good appearance of the surface of substrate 12. This cavity leads to decisive losing of wiring reliability.
Cavity frequently appears when aspect ratio (a/b) of the contact hole C is higher than 1, where a represents a depth, and b represents a diameter of the contact hole C. Generally, a contact hole is made into diameter of less than 1 .mu.m, that is, of submicron range, and recent high density memory devices has smaller diameter, and aspect ratio of them is over 3. This tendency is the same in sputtering of other metals than Al, at wiring process.
The first way to cope this problem is to heat the semiconductor substrate, after sputtering, to the temperature of 500.degree. C. to make Al to melt and flow in the contact hole to fill up. But, this heating method cannot solve the above-described problem, because Al has high viscosity in molten state.
The second way to solve the problem is to install a collimator in the sputtering chamber. FIG. 3 is the similar schematic drawings to FIG. 2. FIG. 3A shows flight directions f of Al particles in a sputtering chamber, and FIG. 3B shows thickness distribution of Al film 14 formed in existence of collimator 13 in the contact hole C. The collimator 13 is installed between the Al target 11 and the substrate 12, and only Al particles that pass through the collimator 13 and have flight directions f' perpendicular to the substrate 12, can reach the substrate 12, as FIG. 3B shows. By this method, enough amount of Al particles can reach the bottom of the minute contact hole C, but thickness of Al film becomes thin at the side wall of the contact hole C, as FIG. 3C shows, causing wiring troubles, and further the collimator 13 is easy to be choked and shortens cycle time of maintenance.
The third way is a so-called long distance sputtering technique, in which the distance between the Al target 11 and the substrate 12 is set in the range of 120 mm to 300 mm, and a pressure in the sputtering chamber is kept relatively so low as 10.sup.-2 Pa, to avoid collision of Al particles, and increase the ratio of Al particles which have the flight direction perpendicular to the substrate 12. When Al target material is sputtered by this technique to the substrate 12 which has relatively larger diameter, it is difficult to keep uniformity of thickness of Al film between the central part and periphery part of the substrate 12.
In addition to the defects on the above described film-forming method, the following defects are found: In common sputtering technique, only 20% to 25% of particles sputtered from target reach the substrate to deposit, and the residue, 80% to 75% of particles sputtered deposits on any other place than the substrate. On the other hand, in the long distance sputtering technique, which uses the collimator 13, deposition ratio to the substrate falls down, and as the result, loss of raw target material increases, productivity of a sputtering apparatus goes down, and sputtering cost increases.
A film forming process by CVD technique also has the same problems in filling up the via hole or contact hole that has high aspect ratio. Further, only 5% to 6% of raw material gases is converted to a film, and loss of raw material in the CVD process is larger than in the sputtering process.
Other than the above described method, a thin metal film forming method by a paste in which ultra fine powder of metal is dispersed in an organic solvent, was disclosed by the Japanese Patent Opening Gazette No. 281783/1991, assigned to the same assignee as that of this application. This discloses a wiring method for a printed-circuit board, a thermal head, or a capacitor, in which powder paste is applied by the screen-printing technique in all of the embodiments disclosed by the Gazzette. In other words, it neither discloses nor suggests a thin metal film forming method to a semiconductor substrate.